The present invention generally relates to a segmentation method, and more particularly to a line region segmentation method in which text or line regions are accurately discriminated from original image containing bilevel areas, continuous tone areas and screened halftone areas.
To reproduce high quality images by a digital copying machine or facsimile machine, it is preferable to use an adaptive image processing method for each of the line regions, continuous halftone regions and screened halftone regions which are contained in original images. As a preliminary step which must be carried out before such an image processing method is applied to each region to reproduce high quality images, it is necessary to accurately discriminate between line regions and screened/continuous halftone regions in original images of documents. Conventionally, a block adaptive thresholding (BAT) method has been used as a segmentation method for segmenting line regions from original images. This BAT method is disclosed, for example, in "Bilevel Rendition Method for Documents Including Gray-Scale and Bilevel Image" by N. Tetsutani et al. contained in a journal published by the Institute of Electronics and Communication Engineers of Japan (IECEJ). In a case in which this BAT method is used, an original image is divided into a plurality of blocks each having a prescribed size, and the maximum intensity (or optical density) level and the minimum intensity level for each of the blocks of the original image are detected. In these blocks, if a block has a difference in optical density between the maximum and minimum intensity levels that is greater than a predetermined threshold, then it is decided that the block is a line region or text region. If a block has a difference in optical density between the maximum and the minimum intensity levels that is not greater than the predetermined threshold, then it is decided that the block is a gray-scale region or picture region other than the text region. In this manner, the conventional BAT method has been used to discriminate between text regions and picture regions in original documents.
However, in a case in which the BAT method is applied, a segmentation of screened halftone areas is not performed, and there is a possibility that screened halftone areas of original images be erroneously determined as text regions. Therefore, in order to achieve accurate segmentation of original images when the BAT method is applied, in addition to the application of the BAT method, an additional segmentation process for segmenting screened halftone areas must be carried out. Thus, it is necessary to carry out a complicated method, and a relatively large, complicated system is required to achieve accurate segmentation, as compared with the conventional segmentation apparatus for performing only the simple BAT method.
Generally speaking, screened halftone areas are difficult to discriminate from non-screened areas when the area factor of screened halftone areas is around 50%. And, even when the BAT method is carried out in combination with the carrying out of the screened/non-screened area segmentation process, it is not easy to achieve accurate segmentation of line images from other images no matter what kinds of original images may be inputted. In addition, to detect the maximum intensity level and the minimum intensity level of each block, it is necessary that multilevel tone signals be directly processed. Thus, the conventional method as described above has a problem in that a relatively large, complicated system must be used to carry out the additional processing of multilevel tone signals when the block adaptive thresholding method is carried out.